Determination of the gas content of liquid metals



Nov. 25, 1958 c. E. RANSLEY DETERMINATION OF THE GAS CONTENT OF LIQUID METALS 2 Sheets-Sheet 1 Filed Aug. 8, 1955 22a 22 20 lb DETERMINATION OF THE GAS CONTENT OF LIQUHID METALS Filed Aug. 8, 1955 Nov. 25, 1958 c. E. RANSLEY 2 Sheets-Sheet 2 United States Patent f DETERMINATION OF THE GAS CONTENT OF LIQUID METALS Charles Eric Ransley, Chesham Bois, England, assignor to The British Aluminium Company Limited, London, England, a British company Application August 8, 1955, Serial No. 527,083 Claims priority, application Great Britain August 10, 1954 30 Claims. ((31. 73-19 This invention relates to improvements in the determination of the gas-content of liquid metals.

It has been proposed to measure the content of dissolved hydrogen in a molten metal by a method which comprises the steps of causing a free surface of the metal to form at a location within the body of the metal and of determining the magnitude of the equilibrium pressure of molecular hydrogen which develops at this free surface. Various forms of apparatus for carrying this method into elfect have been described and although these are in general satisfactory, they have the disadvantages that a relatively long period of time (of the order of 8 minutes) is required to ensure that the end point has been reached, that the indicator employed moves comparatively slowly and that there are occasional failures to respond at all. These drawbacks appear to stem from the blanketing effect of aluminium'oxide films on the transfer of hydrogen from the liquid metal to the free space created within the metal by the apparatus employed and although various methods of clearing this oxide away have been suggested they ,do notalways guarantee success. 7. 7

It is an object of the present invention to provide an improved apparatus for determining the gas-content 'of liquid metals which shall have .a shorter response time and be more reliable in operation than the method and apparatus of the proposals referred to above.

According to one feature of the present invention an apparatus for measuring the content of-hydrogen 'dissolved in a molten metal performs the steps of causing a free surface of the metal to form at a location within the body of metal, bringing a volume of an inert gas into contact with the metal below said free surface, circulating the gas a plurality of times through said location and a path extending externally of the body of the metal and determining the hydrogen-content of the circulated gas when it is substantially in equilibrium with the hydrogencontent of the molten metal.

The inert gas, which may be nitrogen or argon, picks up some hydrogen from the molten metal when it is first brought into contact with the latter and the hydrogencontent of the inert gas increases during each subsequent period of contact until it is in equilibrium with the hydrogen-content of the molten metal. In general, the number of contacts required between the body of inert gas and the molten metal is relatively large, e. g. about 100, before equilibrium is reached and a convenient method of securing this is to circulate a very small volume of the inert gas through a closed path which includes an internal portion in which the gas bubbles through the molten metal and an external portion in which the gas is isolated from the air. The means for determining the hydrogen-content of the circulated gas is connected into the external portion of the closed path. It may comprise a palladium thimble for separating off, in a very small subsidiary volume, the hydrogen contained in the inert gas and means, such as a manometer, for measuring the pressure developed in the subsidiary volume. The

- mersion head 1 are each connected by a suitable small- 2,861,450 Patented Nov. 25, 1958 thimble may be arranged in the part of the apparatus which is to be dipped into the molten metal so that it will be heated to the temperature necessary to promote diffusion of the hydrogen therethrough. The preferred device for measuring or determining the hydrogen-content of the inert gas is, however, one making use of the thermal conductivity of the gas and a suitable device is a Shakespear Katharometer (see Gas Analysis by Measurement of Thermal ConductivityH. A. Daynes Cambridge University Press, 1933).

In order that the invention may be clearly understood, some embodiments thereof will now be described by way of example reference being made to the accompanying drawings in which: 7

Fig. l is a sectional view of one embodiment thereof, and

Fig. 2 shows a modification.

The apparatus comprises, as essential parts, animmersion head indicated generally at 1 by means'of which the insert gas is to be brought into direct contact with the molten metal 2 below the surface thereof; a pump indicated generally at 3 by means of which the gas is to be circulated to and from the'immersion head 1; and means indicated generally at 4 for measuring the concentration of hydrogen in the gas as it circulates externally of the molten metal. a

The total volume of the inert gas which is to'be employed in an actual determination must bevery, small and it has been found that a volumeofl to 2 cc. (at standard temperature and pressure) issuitable.

A suitable material from which to construct-the. immersion head 1.must be one which is (1),not attacked by the molten metal concerned, (2) impermeable-to gas, (3) stable to hydrogen at the temperaturerof the molten metal, even after exposure to air atelevated ,tempera tures, (4) mechanically. robust, and (5) resistant; to thermal shock. Although dense ceramic materials, e. g. of alumina, are capable of use under experimental conditions, they are not suitable for routine use in production work and it has been found that a materials which fulfills all requirements is stainless steel (e. g. of the 18-8 Cr-Ni type) coated with a .vitreousenamel and, preferably, protected with a refractory wash which adheres firmly to the enamel.

In the present example, illustrated in Fig. 1 the immersion head comprises a vertical twin'bore tube 5 (the bores being indicated at 6 and7) which over the lower part of its length is partially cut away so that it only comprises the single bore 7 in this part. A small hood 8 is secured on the lower end of the twin-bore portion of the tube 5 so that the shorter bore 6 opens. into the upper part 9 of the hood-space, this part being separated from the lower part of the space by a partition or dia phragm 10 of porous ceramic material which prevents entry of molten metal into the bore. The longer bore 7 which extends below the skirt 11 of the hood 8by, for example, about 1"-1.5", is pluggedat its lower end with porous ceramic material 12 for the same purpose. The hood 8 may have an internal diameter ofabout 0.8" and a depth of about 0.4, while the bores 6 and 7 in the tube 5 may be about 0.04 indiameter. V

The head 1 when for use with molten aluminium, is constructed from the material referred to above, namely 18-8 heat-resisting stainless steel enamelled with, for example, a vitreous enamel konwn by the trade name Solaramic 5210/30 and coated with a refractory ma: terial which reacts with the enamel at the temperature of molten aluminium and becomes firmly adherent thereto. p

The upper ends of the twin bores 6 and 7 in the imbore tube 13 and 14 respectively (internal diameter from the head 1 so that immersion of the latter in molten metal 2 cannot harm the rest of the equipment. The tube 14 connected to the bore 7 of the headleads into one'end of a small bore passage 15jformedjthrhdghjti copper block 16 adjacent to the cell" 170i o eort'wo substantially identical cells 17 and' '18; formed the this first -'or measuring cell 17communicatingj with he passage 15" through 'a further such passage: 1'19. "Eaph' cell 17 and 18 has supported therein a'tine'pl'ati'rinm w re spiral indicated at20fand 21 respectiv'ely which is insulated from the block 16 and connected'at its endsto lead-out wires passing through a gas'tight seal of'glass' 22 and 23 respectively or the like whichseals" the lower endof a copper cylinder 22a. and 22b respectively secured-in'a'gas-ti'ght manner in the copper-block 36 to close'the ends of the cells17- and 18 respectively; The volume of each cell 17 and 18 may be about 0.5 cc. and the second or comparator cell 18 is in free communication with a small-bore passage 24 formed through the block 16 but in free communication at its ends with the atmosphere.

The other end of the small-bore passage 15 associated with the measuring cell '17 is connected. by another finebore tube '25 through a ball-type outlet valve 26 to the pump 3 which has an extremely small swept'volume, the inlet valve 27 of this pump 3 being arranged to control th'e'pas'sage of gas to the pump the fine-bore tube 13 leading to the upper'end of the shorter bore 6in the im mersion head 1. The last-mentioned tube'13 has a branch connection 29 through which an inert gas may be sup plied, under the control. of acock 30, from a source such as a small cylinder 'of the compressed gas 31. The cock 30 is such'thatwhen closed the gas-filled volume'on the circuit side thereof is extremely small.

A convenient form of pump 3 comprises a, steel disc 32 having the connections from the inletand outlet valves 27'and 26 arranged to open 'into'the deepest part of a shallow'concavity'33 machined in its underside and over which is'secured', in a gas-tight manner, athinsteel dia phragm 34. Means is provided for periodically depressing the central portion of the diaphragm 34 into the concavity 33 and releasing it, for example a plunger 35 operated by a cam 3 6 which is rotatable by means of a crank-handle (notfshowny The volume of the pump chambenrnay be"0.5 cc;

The two platinum spirals 2t and 21 are connected electrically into a normal Wheatstone bridge arrangement with two fixed resistanc es 37 and 38 having a low temperature coefficient, and'a'potential difierence of, say, 1.8'volts is established across the bridge. Under these conditions a current of about 120 milliamperes is passed through each filament 20 and 2 1. Thebridge' is initially balanced by'adjustment of a'subsidiary resistance 39, the total resistance in each, arm being of the order 'of 16 ohms. Should the inert gas,'in'this example, nitrogen, in the measuring cell 17 become richer in'hydro'gemthe filament or spiral 20 in this cellfwill be cooledrelatiyeto that in the comparatorcell'ls and its resistance will be lowered to unbalance the bridge. The most convenient method of measurement is to determine the out of-bal'ance potential across the bridge by means of a millivoltmeter 40 or potentiometer. If the former is used it is preferably an instrument with a high electrical resistance sa ohms) in order that damping of the bridge may be avoided. i

In the operation of the apparatus, nitrogen is admitted to the gas circulation circuit and when the measuring cell 17 is filled with the gas, the supply of current fto the bridge is established and the bridge balanced. i

The cock 30 is opened so thmat nitrogenwill flow through both bores 6 and 7 of the immersion head ,(the inlet and outlet valves 27 and 26 beingso arranged .that they are both opened by the pressure ofthe ga s). zand iss ue as a steady stream from both the hood 8 and the lower end of the bore 7 of the twin-bore tube 5. The head 1 'isimm'ersed 'in'the molten metal 2 with the gas still flowing and after a few seconds the cock 30 is closed and the operation of the pump 3 commenced. Conveniently, the rate of operation is such that the total volume of nitrogen in the circuit is circulated about once every second an, under these conditions, the equilibrium concentration 'of'hydrogen is built up in about 1 /2 minutes.

Where an inert gas other than nitrogen is employed, the comparator cell 18 must be maintained full of this gas at atmospheric pressure during the measuring operations.

The example described with reference to Fig. 1 may be modified asshown in Fig. 2 by replacing the means described tor'measuring the hydrogen content of the inert gas with means for-measuring the pressure of the hydrogen in the inert gas. This means may comprise a thimble 41 of a material, e. g, palladium, which is pervious to hydrogen, but ,is impervious to the inert gas, thethimble 41 being disposed with clearance within the lowerj 'endof the bore 6 of the measuring head 5 and havingits interior connected to a pressure sensitive device, e.; g. a manometer 42. The annular space 43 between the 'thi'mble 41 and the wall of the head 1 defining the bore 6 is connected to the cock 30 and to the inlet valve'27of the previous example and thefine bore tube 14'is connected directly to the tube 25 leading to the outlet valve 26 (the copper block 16 being omitted). The thimble 41 is heated by the molten metal 2 to the temperature necessaryto promote diffusion of hydrogen therethrough.

The operation of the modified apparatus will be readily understood from'the foregoing example.

'Itfwill be understood that the apparatus described is capable ofrnodification in various ways. For example, other types of valves may be. employed in the pump construction injplace of the ballvalves referred to and the pumnelements may be constructed frornnon-corrodible materials such as Phosphor-bronze.

The bores 6 and 7 in the tube S may, if desired, beof the same lengthin which case the lower end of the bore 6 would be sealed and a passage would be provided in the wall of the tube 5 to place the bore 6 in connection with the upper'part 9 of the hood-space.

, What I claim is;

1. Appa'ratus' for determining the gas-content of liquid metals, comprising an immersion head adapted to be immersed within the body of liquid metal, said immersion head including means for bringing a volume of inert gas into' contact with the liquid metal in an area adjacent said head and Within the body ofliquid metal, means providing a path connected with said head for flow of the gas externally of said immersion head, means for circulating the gas a plurality of times through said immersion head and said liquid metal and through said path, and means for determining the hydrogen-content oithe-circulatedgas,

- 2. Apparatus accordingto claim l wherein the immer sion headcomprises' a hollow bodyadapted to be at least'partly immersedin the liquid metal and having at least a part of its walls composed of a material which is pervious to hydrogen and to the inert gas but impervious to the molten metal.

3. Apparatus according to claim 1 wherein the immersion head comprises a hollow body adapted to be at least partly immersed in the liquid metal and having at least a part of its walls composed of a porous ceramic material which is pervious to hydrogen and to the inert gas but impervious to the liquid metal.

4. Apparatus for determining the gas-content of liquid metals, comprising an immersion head adapted to be immersed within the body of liquid metal, said immersion head including Ineanstor bringing a volume of inert gas into contact with the liquidmetal below the surface thereof, and a closed path for now of said inert gas, said path including an internal portion in which the gas bubbles through the liquid metal and an external portion in which the gas is isolatedfrom the air, means for circulating the gas through said path and means for determining the hydrogen-content of the circulated gas.

5. Apparatus according to claim 4 wherein the volume of the external portion of the closed path isv of the order of l to 2 cc.

6. Apparatus according to claim 4 wherein the means for determining the hydrogen-content of the circulated gas is connected to the external portion of the, closed path.

7. Apparatus according to claim 4 wherein the means for determining the hydrogen-content of the circulated gas comprises means for separatingoif, in a very small subsidiary volume, the hydrogen contained in the inert gas and means for measuring the subsidiary volume. r

8. Apparatus according to claim 4 wherein the means for determining the hydrogen-content of the circulated gas comprises a vessel 'for separating 01f, in a very small subsidiary volume, the hydrogen contained in the inert gas, and said vessel has at least a part of its wall composed of a material which is-pervious to hydrogen and impervious to-the inert gas, and means for measuring the pressure developed in the subsidiary volume.

9. Apparatus according to claim 4 wherein the means for determining the hydrogen-content of the circulated gas comprises a vessel for separating off, in a very small subsidiary volume, the hydrogen contained in the inert gas, and said vessel has at least a part of its wall composed of palladium, and means for measuring the pressure developed in the subsidiary volume.

10. Apparatus according to claim 4 wherein the means for determining the hydrogen-content of the circulated gas is connected to the external portion of the closed path and measures the thermal conductivity of the gas.

11. Apparatus according to claim 4 wherein the means for determining the hydrogen-content of the circulated gas is connected to the external portion of the closed path and comprises an electrical resistance wire having a high temperature coei'licient of electrical resistance and means for measuring the electrical resistance of the wire.

12. Apparatus according to claim 4 wherein the means for determining the hydrogen-content of the circulated gas the pressure developed in is connected to the external portion of the closed path and comprises an electrical resistance wire having a high temperature coefiicient of electrical resistance and electrical resistance measuring means comprising an electrical bridge network having the electrical resistance wire connected in one arm thereof.

13. Apparatus according to claim 4 wherein the circulating means comprises a pump having a chamber connected in the external portion of the closed path.

14. Apparatus according to claim 4 wherein the circulating means comprises a pump having a chamber connected in the external portion of the closed path, and two valves are connected in the external portion of the closed path one on each side of the pump chamber, one of said valves being arranged to close and the other to open on the pumping stroke of the pump whereby the gas is circulated in one direction only in said closed path.

15. Apparatus in accordance with claim 4 wherein a supply of inert gas is connected to the external portion of said closed path.

16. Apparatus according to claim 4, wherein the circulating means comprises a pump having a chamber con nected in the external portion of the closed path and two valves are connected in the external portion of the closed path and a supply of inert gas is connected to the external portion of the closed path, the valves being positioned on the same side of the supply of inert gas and being opened by the pressure of said gas flowing from the supply thereof.

17. Apparatus for determining the gas-content of liquid metals, comprising an immersion head adapted to be immersed within the body of liquid metal, said immersion head including a pair of open ended tubes, a hood fixed to said tubes, a partition of porous ceramic material mounted within said hood and separating the hood space into an upper area and a lower area to prevent entry of liquid metal into said upper area, one of said tubes opening into said upper areaand the other tube opening into the liquid metal and the open end of said other tube being plugged with a ceramic material to prevent entry of liquid metal into the tube, said tubes being connected with a closed circulatory path for bringing an inert gas thereinto for flow therethrough and through the liquid metal, means for circulating said gas through said path and tubes and means for determining the hydrogencontent of the circulated gas.

18. Apparatus for determining the gas-content of liquid metals, comprising an immersion head adapted to be inserted in the liquid metal, said immersion head including a pair of open ended tubes, one of said tubes being longer than the other of said tubes, a hood fixed to said tubes, a partition of porous ceramic materialmounted within said hood and separating the hood space into an upper area and'a lower area to .prevent entry of liquid metal into said upper area, the. shorter of said tubes opening into said upper area and the longer of said tubes extending and opening into the liquid metal and the open end of said longer tube being plugged with a ceramic material to prevent entry of liquid metal into said tube, said tubes being connected with a closed circulatory path for bringing aninert gas thereinto for flow therethrough and through the liquid metal, means for circulating said gas through said path and tubes and means for determining the hydrogen-content of the circulated gas.

19. Apparatus according to claim 18 wherein said hood provides a skirt and the longer tube extends into the liquid metal below said skirt.

20. Apparatus according to claim 18 wherein said hood provides a skirt and the longer tube extends into the liquid metal below the skirt a distance of about 1"-1.5, the hood having an internal diameter of approximately 0.8 and a depth of about 0.4" and the internal diameters of the tubes are about 0.04".

21. Apparatus according to claim 18 wherein the immersion head is constructed from 18-8 heat-resisting stainless steel.

22. Apparatus according to claim 18 wherein the im mersion head is constructed from 18-8 heat-resisting stainless steel enamelled with a vitreous enamel.

23. Apparatus according to claim 18 wherein the immersion head is constructed from 188 heat-resisting stainless steel enamelled with a vitreous enamel and the enamel is coated with a refractory material which re acts With enamel at the temperature of the liquid metal andbecomes firmly adherent thereto.

24. Apparatus for determining the gas-content of liquid metals, comprising an immersion head adapted to be immersed within the body of liquid metal, said immersion head including a pair of open ended tubes, one of said tubes being longer than the other of said tubes, a hood fixed to said tubes, a partition of porous ceramic material mounted within said hood and separating the hood space into an upper area and a lower area to prevent entry of liquid metal into said upper area, the shorter of said tubes opening into said upper area and the longer of said tubes extending and opening into the liquid metalv and the open end of said longer tube being plugged with a porous ceramic material to prevent the entry of liquid metal thereinto, said tubes being connected with a closed circulatory path including a pump having a pump chamber for circulating an inert gas a plurality of times through said path, said tubes and the liquid metal, the shorter tube being connected at its other end to the pump chamber by way of a first valve which is arranged to close on the pumping stroke of the pump, and a cell for measuring the hydrogen-content of the gas, and a second valve, thepump chamber heing connccted by way of the, second valve which. is. arranged toflopen. on the pumping stroke of; thepump with the cell and the, cell being,connected touthe. other,:end.o f the longentube.

25. Apparatus accordingtoclaim 24 wherein a sup: ply of. inert. gas is connectible 1 into, the closed circulatory path, which, is connected I to the other end of. the. shorter tube and said two valvesarecausedto open bythaprcs: sureof the. inertgas when, it is, connected into ,the closed circulatory path.

26. Apparatus according. to claim 24 wherein said pump comprisesuasteel disc formed with aconcavityon oneside and having an ,inletpOnnected to \said first valve and opening into the concavity and having an outlet connectedto saidsecond valve and opening into the concavity, and a thin steel daphragnl. secured in a gas-tight manneruovern the concayitml hc spa e. between the diaphragm andtheiface. of =the, ldisc defining the. concavity constituting; the pump. chamberoand; thev central .p r ion Of ap r gm. cing rrangedto beperiodically depressed into the .cavity, and meansfqrdepressing the central portionof the diaphragminto the concavity.

27. Apparatus according to claim 2,4 wherein.the,vo1- .umepf the pump, chamber is of the order of 0.5 cc.

28. Apparatus according. to ,c1aim24 wherein a platimm wire spiral is mounted in the cell and is connected in onetarm. of. a. bridge network.

29. Apparatus according tocclaim. 24, whereinthe vol umeot the .measuringcellisof theordenof 0.5 cc.

:30. *Apparatus according.to.claim.24 whereina plati? -num.wire spirahismounted in. thecelland is connected in one armot albridgenetwork anda comparator, cell is provideiwhichvis. substantially identical with the. aforementioned. cell, .aplatinum wire spiral: being mounted in said comparator cell and connected in another, arm of saidbridgenetwork.

. ReferenceseCitedg-inthe .file .of this patent UNITED STATES PATENTS ,47 2 92 1,615,299 1927 2.140.60 93 2,514,690 1950 2 6 45.117; 1953 zeal- & 7 1954 P,.-.-:-'-:--t-'----.--- 5 FOREIGN PATENTS 09,2 8 Ge m n .-.-.-J--: -r=-= y 17, 3 5 4.865 Gr at;Br .in--499-999:- 5 

